Soluble silicates combined with various setting agents have been used in the chemical fixation and solidification (CFS) field since the 1960's. Perhaps the most notable patent in this area is U.S. Pat. No. 3,837,872 owned by Chemfix Technologies Inc. although there is a great deal of other very similar prior art. A good review of prior art techniques is given in U.S. Pat. No. 4,600,514. None of the prior art, however, teaches the in situ formation of soluble silicates from solidification additives and/or wastes. One major drawback to the Portland cement/soluble silicate process has been that the reaction is often so rapid that control is difficult especially in batch treatment systems. For this reason, the process has been used primarily in large scale, continuous flow applications. Another disadvantage is that the additives, i.e. a solid (usually Portland cement) and a liquid (usually a 38% solution of 3.22SiO.sub.2 :Na.sub.2 O sodium silicate), must be separately stored and added to the waste with very rapid and complete mixing. These problems were addressed in two patents, U.S. Pat. Nos. 4,518,508 and 4,600,514, assigned to SolidTek Systems Inc. and Chem-Technics, Inc., respectively. In these inventions, solid soluble silicates were substituted wholly or partially for the liquid solution using other means to increase the viscosity of low-solids wastes so that phase separation would not occur while the mixture hardened in a slower and more controlled manner.
Much use has been made of soluble silicates in CFS technology to fix toxic metals in less soluble forms that will meet RCRA standards for leachability. However, when the metals are already speciated as partially soluble compounds, such as hydroxides, the silicates do little other than to reduce access of the leachant to the metals by physical means. The reason for this is that the silicate is quickly used up by its reactions with the setting agent (Portland cement, etc.) and/or the waste, and is not available later as the metal compounds slowly dissolve.